Product standards require that all safety critical components of a product meet the requirements of the relevant component standards. This means that the safety of components must be assured before that of the product itself.

What Makes Components Safety Critical?

A component is classified safety critical, for example, when its failure may cause danger. As a general rule, it can be said that all functional, protective or insulating components connected to the mains voltage are considered safety critical. The following describes safety critical components, with a few examples.


Transformers in accordance with their name transform main voltage (= primary voltage) to suitable secondary voltage. Secondary voltage is often accessible by hand, so the winding of the primary circuit must be well insulated from the winding of the secondary circuit. The primary and secondary windings themselves may be insulated with appropriate insulation coating, in which case they may even be in contact with each other in the transformer pack. In most transformers, however, only one of the windings is insulated by coating, so the windings are not allowed to touch each other. Enameling of the winding is not considered sufficient insulation, as it may not be durable enough against thermal and mechanical stress occurring during the life cycle of the transformer. In such a case, the transformer itself and its manufacturing process must be extremely well planned, so that the windings do not come into contact with each other. Insufficient safety distance between the windings may cause an electric shock to the user when a dangerous mains voltage (e.g. 230 VAC) can interfere with accessible secondary voltage (e.g. 12VDC).


Switches may fail after repeated use in such a way that the contacts get welded together causing a fire hazard. The safety of switches is tested for example with the endurance test, where the switch is exposed to tens of thousands of periods of use, according to its design values. To pass this test the switch must still be in working condition after the test.
Terminal blocks

The insulation material of terminal blocks comes into contact with live parts, which means that they will have to resist the tracking and heat stress that occurs during use. These properties are tested with tracking and glow wire tests. In addition, the live parts of the terminal block must be sufficiently well insulated from contacts.

How To Choose Safe Components?

First, the suitability of a component for the intended use and conditions must be ensured. Design values must be taken into account, so that the component will not be loaded beyond its nominal values. Also, it should be ensured that a component is suitable for the intended conditions (temperature, moisture, pollution) that are prevailing, both in the final product and in the environment where the product will be used.
After assessing the suitability of a component, it must be ensured that it meets the safety requirements. The best way to ensure conformity is to select components that are manufactured by well-known manufacturers, certified by an accredited certification company, and awarded an appropriate certification mark license (e.g. FI mark). Certification ensures that the product has been properly tested by an independent testing laboratory according to the relevant safety standards and that compliance of the product is monitored by annual audit of the manufacturing site.
If it has not been possible to verify the conformity of a component, it has to be done, at the latest, while ensuring the conformity of the end product. This, however, is time-consuming at this stage, as often the safety requirements of the component demand more comprehensive testing (e.g. long endurance tests) than safety requirements of the end product.

Alternative Components

During a product’s life cycle, the manufacturer of an electrical appliance may need to use other similar components in production. Manufacturer’s can prepare for such a situation as early as the design stage, by selecting alternative components. In this case, the manufacturer already has a safe alternative to replace the actual component. The safety of alternative components should be ensured in the same way as for the actual component.

How Can SGS Help You To Choose Components?

Our experts can help you to identify the safety-critical components in your electrical device. SGS can also estimate whether components have been certified by a competent certification company according to the relevant safety standards and whether they are suitable for the intended use, for example the correct operating voltage, operating conditions, etc.

For more information:

Kari Vesterinen
Product Line Manager, Safety Testing
SGS Fimko Oy
t: +358 9 6963418